At call set-up, a mobile station is assigned a traffic channel which means that a mobile station is actually moved from a control channel to a traffic channel. Generally a mobile station is always assigned a traffic channel in the same cell as the control channel except when a cell fulfilling some given network requirements (in a hierarchical cell structure) exists or when there is traffic channel congestion in the cell in which the mobile station is connected to a control channel. A hierarchical cell structure is often used to gain capacity in a mobile radio network in a hierarchical cell structure cells of different sizes will cover one and the same geographical area and the advantage thereof is that frequencies can be reused with a closer reuse pattern.
When a mobile station is in an idle mode, it performs measurements on all control channels ( in the PDC (Personal Digital Cellular) system the control channels are denoted perch channels) in the area in which it is located or within the relevant location area. The control channel having the highest signal strength is chosen and it belongs to the cell that the mobile station will use at call set-up, also called the access cell. The mobile station then listens to broadcasting information sent out in a broadcasting information message on the selected control channel. The broadcasting information message among other information contains information about the number of reporting zones at call set-up. This information is thus (together with other information) received by the mobile station. The mobile station will then send signal strength information and control channel number, using the idle state measurements, to the system at call set-up for as many neighbouring cells as the information relating to the number of reporting zones has specified. In for example one implementation of the PDC-system two neighbouring cells are specified but the PDC standard as such e.g. allows twenty neighbouring cells to be specified.
When there is a high traffic load in a cell so called cell load sharing may be implemented. This means that the cell borders are moved through changing the handover offset parameter that is defined for a cell-neighbouring cell relation. Then mobile stations which are connected to a traffic channel in the cell having a high traffic load are made to perform a handover to the neighbouring cell which has a lower traffic load.
U.S. Pat. No. 5,241,685 relates to the provision of load balancing through mechanically moving the borders between any two cells such that an overloaded cell becomes smaller whereas the neighbouring cell becomes larger. According to this document this is achieved through lowering the entering signal strength threshold for handover to the neighbouring cell and/or increasing the entering signal strengh threshold for handover from the neighbouring cells, wherein the thresholds are unique for any two cells.
Assignment to a neighbouring cell is only activated when congestion occurs in a cell. However, when there is a request for a resource (i.e. a traffic channel) for a new call in a cell which is congested, the requirements for assignment to a neighbouring cell, might not be fulfilled. These requirements are based on (handover) parameters determining whether a mobile station is within the handover area between cells or not. Thus, if these requirements are not fulfilled such calls will be disconnected. Thus, congestion situations easily arise leading to the disconnection of calls. This is a serious drawback. Consequently an assignment to an assignment candidate forming an alternative to the access cell is only performed if there are no more resources in the access cell, in other words, there is congestion. The probability that a mobile station requesting resources in a congested cell is located in the handover area between two cells is quite low which means that the probability that the call be dropped is high.